In well logging of earth boreholes there exist applications where it is necessary to accurately measure, in a hostile and noisy environment, very small electromagnetic signals. One such application, which has had limited practical success to date, is a logging device which measures nuclear magnetic resonance ("NMR") properties of earth formations. It has been recognized that particles of a formation having magnetic spin, for example atomic nuclei, protons, or electrons, have a tendency to align with a static magnetic field B.sub.0 which is imposed on the formation. If a pulse of alternating current having a frequency f is passed through a transmitter coil, producing an oscillating polarizing field B.sub.1 perpendicular to the static field B.sub.0, a population of protons in a formation would be tipped away from the static field direction. At the end of the pulse, when B.sub.1 is removed, the protons precess about the B.sub.0 vector. After a characteristic time called the longitudinal or spin-lattice relaxation time T.sub.1, the protons have relaxed to thermal equilibrium, wherein a percentage of protons are aligned in the direction of B.sub.0.
Even in ideal conditions, the accurate measurement of these phenomena requires sensitive equipment. Measurements of NMR characteristics of rock samples can be made in a laboratory with reasonable accuracy, but the making of comparable measurements in an earth borehole is rendered more difficult by the hostile environment where temperatures may reach several hundred degrees Fahrenheit, pressures may reach thousands of p.s.i. and all of the equipment must be packed within a cylindrical volume of only several inches in diameter. An improved well logging apparatus for measuring NMR characteristics is set forth in U.S. Pat. No. 4,933,638, assigned to the same assignee as the present application. Reference can be made to said U.S. Pat. No. 4,933,638 for a description of prior art NMR logging approaches. The improved logging apparatus of said patent is summarized in conjunction with FIG. 1-3. In FIG. 1, a borehole 10 is shown adjacent to formations 11, the characteristics of which are to be determined. Within borehole 10 is a logging tool 13 in accordance with the referenced U.S. Pat. No. 4,933,638, which is connected via a wireline 8 to surface equipment 7. Tool 13 has a face 14 shaped to intimately contact the borehole wall, with minimal gaps or standoff, and a retractable arm 15 which can be activated to press the body of the tool 13 against the borehole wall during a logging run, with the face 14 pressed against the wall's surface. A mudcake 16 is shown on the borehole wall. As described in the referenced patent, although the tool 13 is shown as a single body, the tool may alternatively comprise separate components such as a cartridge, sonde or skid, and the tool may be combinable with other logging tools. Also, while a wireline is illustrated, alternative forms of physical support and communicating link can be used, for example in a measurement while drilling system. The tool 13 includes a magnet array 17 and an antenna 18 positioned between the array 17 and the wall engaging face 14. Magnet array 17 produces a static magnetic field B.sub.0 in regions surrounding the tool 13. The antenna 18 produces, at selected times, an oscillating magnetic field B.sub.1 which is focussed into formation 12, and is superposed on the static field B.sub.0 within those parts of formation opposite the face 14. The "volume of investigation" of the tool, shown in dotted lines in FIG. 1, is a vertically elongated region directly in front of tool face 14 in which the magnetic field produced by the magnet array 17 is substantially homogeneous and the spatial gradient thereof is approximately zero. As described in the referenced patent, the tool 13 makes a measurement by magnetically tipping the nuclear spins of particles in formation 12 with a pulse of oscillating field B.sub.1, and then detecting the precession of the tipped particles in the static, homogeneous field B.sub.0 within the volume of investigation over a period of time.
FIG. 2 shows a magnet array 17 disclosed in an embodiment of the referenced patent. The magnet array includes three permanent magnets 24, 25, 26, which are mounted parallel to each other within a metal alloy body 27. The body 27 should be of a material having low magnetic permeability, so as to not interfere with the static magnetic field. Magnets 24, 25, 26 are elongated in the longitudinal direction of the borehole. The magnetic poles of each magnet are not on the smallest faces of the slab, commonly viewed as the ends of a bar magnet; instead, the poles appear on the two opposing edges of the slab magnet and point to the left and right, respectively. Therefore, within the formation 12, the magnetic field B.sub.0 surrounding the magnets remains fairly constant along the longitudinal direction of the borehole axis. In the illustration of FIG. 2, magnets 24, 26 are symmetrically mounted in the two sides of the body 27 with the north poles facing the same directions. Magnet 25 is positioned parallel to and between the other two magnets, but with its north poles facing oppositely from magnets 24, 26. Magnet 25 is also shifted slightly away from face 14, relative to magnets 24, 26. The north poles of magnets 24, 26 point in the direction of the face 14 of the tool, while the north pole of magnet 25 is pointed away from the face 14. The central magnet may alternatively be reversed or omitted.
As described in the referenced patent, the metal body 27 has, on the front face 14 thereof, a semi-cylindrically shaped cavity or slot 28 which faces formations engaged by the face 14. The cavity 28 is adapted for receiving an RF antenna 18 that is shown in FIG. 3. The antenna is positioned outside of the metal body 27 (FIG. 2) of the tool, and is thereby shielded from electromagnetic communication with regions of the borehole which lie behind the body 27, or regions of other formations in directions intercepted by the body 27. Antenna 18 is thus responsive only to magnetic fields originating in front of the wall engaging face 14, e.g. fields originating in the formation 12 or in the mudcake or mud which contacts face 14 in the vicinity of the antenna 18. In a disclosed embodiment of the referenced patent, the body 27 is made of metal alloy sheathing, rigidly attached to interior metal bracing, which envelops most components of the tool other than the antenna 18, including the circuitry, the magnet array 17, and the hydraulics system of the arm 15. The patent points out that the body 27 can alternatively be constructed of other materials, so long as the overall structure is sufficiently strong and the magnetic field of the magnet array 17 can penetrate the body and enter the adjoining formation 12.
In the referenced patent, antenna 18 is used both as an RF transmitter to produce an oscillating magnetic field in formation 12, and as a receiving antenna to detect coherent magnetic signals emanating from precessing protons immediately after the oscillating field is terminated. The antenna serves effectively as a current loop which produces an oscillating field B.sub.1 within the volume of investigation that is perpendicular to B.sub.0. The antenna 18 comprises a highly conductive semi-cylindrical cavity or trough 29, end plates 30, 31 and center conductor or probe 32 which extends from one end plate 30 to the other end plate 31, parallel to and centered in the semi-cylindrical trough 29. The trough 29, end plates 30, 31 and antenna probe element 32 are indicated as preferably being made of heavy gauge copper which has very low electrical resistance. Antenna probe element 32 is insulated from end plate 30 by a non-conducting bushing 33 and is connected to a conductor 34 on the other side of end plate 30. Probe 32 is attached at its other end to the other end plate 31 so that current passes freely between trough 29 and probe 32 via end plate 31. Conductor 34 is shown in FIG. 3 schematically as being connected to circuitry including an amplifier 35 and a detector 36. All connections in antenna 18 are stated to be brazed or silver soldered, to ensure a suitably low resistive loss. As described in the referenced patent, RF antenna 18 can be driven by amplifier 35 during specified periods of time (the signal being applied at conductor 34 with respect to the antenna body), during which it serves as an RF antenna transmitter. Alternatively, at other specified times, antenna 18 is electronically connected to detector 36, during which time it serves as an RF receiving antenna. In some modes of operation, antenna 18 may be called upon to alternately function as transmitter or receiver in very rapid succession. The space between trough 29 and antenna element 32 is stated to be preferably filled with a ferrite. Several tuning capacitors 38 are connected between the base of antenna element 32 and the trough 29, with the capacitances thereof being chosen to produce an LC circuit, with the resonant frequency being the Larmor frequency. Reference can be made to the U.S. Pat. No. 4,933,638 for further disclosure regarding dimensions of the antenna, the signal-to-noise ratio of signals detected by the antenna, and other details.
It is among the objects of the present invention to provide an improved antenna useful in the described type of logging device and other logging devices. It is also among the objects of the present invention to provide improved wear plates for the described type of logging device and other logging devices.